As a secondary battery having high energy density, a non-aqueous electrolyte lithium secondary battery using a carbonaceous material as a negative electrode has been proposed (see, for example, Patent Literature 1 to 4). This utilizes the fact that a lithium-intercalated carbon can be easily formed electrochemically. By charging this battery, the lithium in a positive electrode made of a chalcogen compound such as LiCoO2 is electrochemically doped in between layers of negative electrode carbon. The lithium-doped carbon acts as a lithium electrode, and by discharging, the lithium is de-doped from between the carbon layers and is returned into a positive electrode. Since such a non-aqueous electrolyte secondary battery is small and light, and has a high energy density, demand for the battery to be used as a power source of mobile devices is expanding in recent years.
As a negative electrode material for such non-aqueous electrolyte secondary batteries used in small mobile devices, non-graphitizable carbon (also called “hard carbon”) having a discharge capacity of much more than 372 mAh/g carbon, which is the theoretical value of graphite, has been widely used. To increase the capacity, for example, a method, comprising calcinating the non-graphitizable carbon under a condition of flow of inert gas (Patent Literature 5) or under reduced pressure (Patent Literature 6) to remove gas generated in the carbonizing reaction, thereby enhancing development of pores (open pores), has been employed.
However, there was a problem in relation to the above method in that, when the thus formed non-graphitizable carbon is left to stand in the air, an oxidation reaction occurs, resulting in increase in an irreversible capacity or deterioration of cycle performances. As a solution for the problem, a method comprising storing the non-graphitizable carbon in an inert gas atmosphere (Patent Literature 7) has been proposed.
As another method for reducing deterioration of the characteristics, a method comprising depositing a thermally decomposed carbon on a carbon surface to adjust the pore size has been proposed (Patent Literature 8). However, in order to increase a discharge capacity, a large number of open pores are desired.
As a novel use exploiting the small and light character of the non-aqueous electrolyte secondary battery, those used for electric vehicles (EVs) driven by a motor alone and for hybrid electric vehicles (HEVS) combining an internal-combustion engine and a motor has been studied intensively. Among others, HEVs combining an engine and a battery-driven motor are now drawing much attention as cars achieving both economic efficiency and low fuel consumption. Especially, as for HEVs, since the start of their commercial sale in 1997, their environment-friendly concept has been accepted, and their market has been growing year by year. In these cars, since the weight saving, the input density and output density of the battery effect an improvement in fuel consumption, improvements in the characteristics of the battery to be mounted on the cars has been demanded. Some of the non-aqueous electrolyte secondary batteries are already in use for cars.
When HEVs run at a low speed or under a driving mode of low load, a load to a motor having a higher efficiency than an internal combustion engine is increased, and when the HEVs run under a driving mode at a high speed or under a high load, such as the occasion making a quick start or passing at a high speed, the load to the engine having a higher efficiency than the motor is increased. Further, when the cars brake, the motor is used as a generator, and a regenerated electric current is stored in a battery. Thus, the motor is mainly used for start, acceleration and slowdown of HEVs. It is demanded that a battery which supplies electric power to the motor does not have a high energy density to supply small electric power for a long period of time as demanded for a small mobile device, but a high input and output characteristics for supplying and receiving a large electric power in a short time.
Thus, the characteristics demanded for secondary batteries of HEVs, are different from those demanded for secondary batteries of a small mobile device. Most of the studies on negative electrode materials for non-aqueous electrolyte secondary batteries has been directed to making improvements in the characteristics of negative electrode materials of secondary batteries used as an electric source for small mobile devices. However, a negative electrode material having the necessary characteristics for a large current input/output non-aqueous electrolyte secondary batteries such as secondary batteries for HEVs has not been studied.
As a negative electrode material for non-aqueous electrolyte secondary battery having a high durability when used for a long time, and which is capable of high current input/output, a non-graphitizable carbon material whose average interlayer spacing d 002 is larger than that of graphite, and which has a character that an electric potential changes depending on an amount of doped lithium, is suitable, and various materials have been proposed (Patent Literature 9).
These materials have micropores. Since moisture is adsorbed in the micropores, there are problems in that an irreversible capacity is increased in initial charging, and a capacity of the battery is decreased with progress of a charge-discharge cycle. To address this problem, a method, wherein the carbon material which is once calcinated is subjected to heat treatment again so as to eliminate the adsorbed moisture, has been proposed (Patent Literature 10). However, since the moisture-adsorbing property itself of the material is not improved, there are problems in that care should be taken for the moisture absorption of the material during storage and production; and control of atmosphere is required.
To reduce molecular adsorption of a material, a material in which a thermally decomposed carbon layer is formed on the surface of the carbon particles by a chemical deposition treatment, has been proposed (Patent Literature 11). However, in large scale production, since the thermally decomposed carbon serves as a binder of carbon particles, there is a problem with regard to the production process in that the material coagulates in a reaction bath, or it is difficult to uniformly form the thermally decomposed carbon layer on a large number of particles. There is also a problem in that increase of cost for production cannot be avoided because of addition of a production step.    Patent Literature 1: JP 57-208079 A    Patent Literature 2: JP 62-90863 A    Patent Literature 3: JP 62-122066 A    Patent Literature 4: JP 2-66856 A    Patent Literature 5: JP 3399015 B    Patent Literature 6: JP 3427577 B    Patent Literature 7: JP 8-298111 A    Patent Literature 8: JP 2003-3238911 A    Patent Literature 9: PCT/JP2005/005908    Patent Literature 10: JP 3619799 B    Patent Literature 11: JP 9-326254 A    Patent Literature 12: JP 3496901 B    Patent Literature 13: JP 7-122300 A